Method of coprecipitating hexagonal ferrites

ABSTRACT

A METHOD IS DISCLOSED FOR COPRECIPITATING CHEMICAL CONSTITUENTS OF FERRITES AS INTIMATELY MIXED, FINELY DIVIDED PARTICLES WHICH MAY BE READILY CALCINED TO FORM FERRIMAGNETIC BODIES. IN A PREFERRED EMBOIDIMENT, IRON IS PRECIPITATED AS ITS HYDROXIDE FROM AQUEOUS SOLUTION AND BARIUM, STRONTIUM OR LEAD IS COPRECIPITATED THEREWITH AS THE RESPECTIVE LAURATE SALT. THE FINELY DIVIDED PRECIPITATED PARTICLES ARE INITMATE MIXTURES OF THE IRON AND COPRECIPITATED ELEMENT SO TYHAT UPON THE DRYING AND CALCINING A DENSE, HEXAGONAL FERRITE OF SINGLE DOMAINSIZE IS FORMED.

Jan. 11, 1972 A. L. MICHELI METHOD OF COPRECIPITATING HEXAGONAL FERRITESFiled June 16, 1969 FERRIC SALT AND OR LEAD PREPARE AQUEOUS SOLUTIONSALT OF BARIUM, STRONTIUM PREPARE AQUEOUS SOLUTION FATTY ACID INAMMONIUM HYDROXIDE MIX COPRECIPITATION OF IRON HYDROXIDE WITH FATTY ACIDSALT OF BARIUM STRONTIUM OR LEAD FILTER,WASH

AND DRY PRECIPITATE BURN OUT ORGANIC MATTER LEAVING FERRIC OXIDE ANDOXIDE OF BARIUM STRONTIUM OR LEAD CALCINE TO FORM HEXAGONAL FERRITEPARTICLES OF SINGLE DOMAIN SIZE ATTORNEY United States Patent C M3,634,254 METHOD OF COPRECIPTTATING HEXAGONAL FERRITES Adolph L.Micheli, Royal Oak, Mich, assignor to General Motors Corporation,Detroit, Mich. Filed June 16, 1969, Ser. No. 833,447 Int. Cl. CM-b 35/26US. Cl. 252-6263 3 Claims CT OF DISCLOSURE This invention relates to themanufacture of ferrites and more particularly it is related to a methodof chemically coprecipitating iron hydroxide and a long chain fatty acidsalt of barium, strontium or lead as finely divided particles which maybe readily converted into dense, highly eflicient ferrites.

While ferromagnetism is commonly recognized as a property of certainmetals, a similar effect known as ferrimagnetism has been observed incertain compounds of metals such as the ferrites. It is known that thereaction products of oxides of iron and oxides of other suitable metalspossess ferrimagnetic properties. These complex oxides are known asferrites and, depending upon the composition and particular treatment ofthe ferrites, their magnetic properties may be either soft or hard.Since some commercially important ferrite compositions do not occur innature, it is necessary that they be manufactured. Ferrites have beenformed by mechanically mixing powders of Fe O and oxides or carbonatesof other metals such as cobalt, manganese, zinc, magnesium, barium, andstrontium and lead. If mixed in the proper known proportions andsuitably sintered, the resulting particles are ferrimagnetic. However,it is realized that the method of forming ferrites by mechanicallymixing the oxide constituents produces a product having magneticproperties less desirable than those theoretically obtainable. This hasbeen attributed, in part, to incomplete mixing which may cause a productof slightly low density to be formed. Moreover, it is quite diflicult toproduce ferrite particles uniformly of single domain size by mechanicalmeans. Although the mechanically formed powders may be milled, it isextremely difficult to achieve uniform powder mixture of about onemicron particle size which is approximately the single domain size ofhexagonal ferrites.

A very useful and commercially important class of ferrites are thehexagonal ferrites such as barium ferrite,

ideally BaFe O (or BaO-6Fe O strontium ferrite (SrFe O and lead ferrite(PbFe O They are hard ferrites which means that they are diflicult todemagnetize. In order for these hexagonal ferrites to possess nearoptimum magnetic properties the individual particle size must closelyapproximate the single domain size which is believed to be approximatelyone micron. Unfortunately, a domain size of this dimension is within therange where hard ferrite precursor powders fabricated by conventionalmechanical techniques normally have their mean particle diameter. In thefabrication of highly Patented Jan. ll, 1972 effective permanent ferritemagnets it is important to be below this critical diameter in thepreparation of the ferrite precursor powders because of subsequent graingrowth during the sintering operation. Since it is extremely diflicultto classify or separate particles of micron size, commercial sinteredhexagonal ferrites typically contain grains which are larger than thesingle domain size and therefore leave something to be desired as far asmagnetic properties are concerned.

It is an object of the present invention to provide a useful, readilyperformed chemical coprecipitation process yielding finely dividedferrite precursor powder containing an intimate mixture of a suitablecompound of iron and of an element selected from the group consisting ofbarium, strontium and lead. The precipitated powder is calcined to formferrite particles of substantially critical domain size or slightlysmaller.

In accordance with a preferred embodiment of my invention a hexagonalferrite such as barium ferrite, strontium ferrite or lead ferrite isformed by a chemical coprecipitation technique. An aqueous solution ofsuitable proportions of a suitable iron salt and a suitable salt ofbarium (or strontium or lead) is prepared. A second aqueous solution ofammonium hydroxide and the ammonium salt of a fatty acid such as lauricacid or stearic acid is formed. The composition of the ammoniumhydroxide-ammonium salt of fatty acid solution is adjusted so that thereis just enough of the fatty acid salt present to precipitate barium, theiron being precipitated by the ammonium hydroxide. The two solutions aremixed at room temperature and a very finely divided precipitate isformed. The precipitate particles each contain an intimate mixture ofiron hydroxide and barium-fatty acid salt. The proportion of iron andbarium in the precipitate is the same as that initially present in thesolution. The precipitate is filtered from the mother liquor, washedwith water and dried. The precipitated powder is heated to decompose thebarium-fatty acid salt and yields barium oxide. The burned out powder isthen calcined to form ferrite particles which are substantiallyuniformly one micron particle size. These particles are found to haveexcellent magnetic properties very closely approaching those determinedtheoretically obtainable in dense crystals of the ferrite of domainsize.

Other objects and advantages of my invention will become more apparentfrom a detailed description thereof which follows.

The drawing is a flow diagram depicting basic steps in a preferredembodiment of the process which have been described briefly and will nowbe described in more detail.

A specific example of the invention will more clearly describe apreferred technique of carrying out my method. A water solution ofreagent grade barium nitrate and ferric nitrate of the correct molarratio lBa++ to 12Fe+ was prepared. The solution comprised 155 grams ofFe(NO -9H O, 8.55 grams of Ba(NO and sufiicient distilled water toprovide a total solution volume of 500 ml. A second water solution oflauric acid dissolved in ammonium hydroxide was prepared. Twenty (20)grams of lauric acid and grams (112 ml.) of reagent grade ammoniumhydroxide solution (28%-30% NH were added to a sufiicient quantity ofdistilled water to provide a clear solution having a total volume of2000 ml. and a pH of about 10.8. The water solution of barium nitrateand ferric nitrate was added to the ammonium laurateammonium hydroxidesolution to completely precipitate all the barium and iron. The solutionof ammonium laurate-ammonium hydroxide was rapidly stirred during theprecipitation and the pH accurately monitored. The precipitation shouldnot be continued below a pH of 8 3 as barium laurate is not suitablyprecipitated below this pH value. Preferably the precipitation isaccomplished at a pH of 8.5.

The brown precipitate was filtered at room temperature from the motherliquor and washed four times with distilled water to remove solubleammonium salts formed during the double decomposition reactions. Thefiltered and washed precipitate was then dried at 200 'F. in airovernight. The dry precipitate powder was analyzed and found to consistof barium laurate and hydrated ferric oxide. The finely dividedprecipitate was heated at 200 250 C. to burn out or decompose the bariumlaurate in the mixture to barium oxide and to fully dehydrate the ferricoxide. X-ray analysis of the fine powder revealed a fine particlemixture of magnetic Fe O and BaO. The particle size of powder at thispoint was in the range of 100 to 500 angstroms. Small portions of thebarium oxide-magnetic ferric oxide powder were calcined at varyingtemperatures between 570 C. and l190 C. for one hour each. Measurementswere then made on each of the specimens with a magnetometer at roomtemperature to evaluate the hysteresis properties of the respectivecalcined specimens. In connection with these measurements the maximumintrinsic coercive force of each of the specimens was noted. In thetable is summarized the maximum intrinsic coercive force for each of thespecimens which had been heated for one hour at the stated temperature.

For uniform heating periods of one hour it was found that a temperatureof approximately 600 C. was required to form a hexagonal ferrite. Thiswas determined by X-ray analysis. It is noted in the above table thatthe values of maximum intrinsic coercive force increased as thecalcining temperatures were increased up to a temperature of about 970C. This appears to be due to the fact that the uncalcined powder wasconsiderably smaller than single domain size for barium ferrites (aboutone micron or angstroms) and some grain growth was necessary to achievethe required domain size. In fact, the specimens which were calcined attemperatures substantially below 970 C. demonstrated superparamagneticproperties which was apparently due to their ultra fine particle size.Superpararnagnetism in these specimens was determined by a Mossbauerstudy. However, the calcining of the powder at 970 C. for one hourproduced barium ferrite particles of approximately single domain sizehaving a maximum intrinsic coercive force of S500 oe. This value ofintrinsic coercive force is in good agreement with that reported in theliterature for single domain barium ferrite powder (H =5350 oe.). It isalso in the range which fits a theoretical value for single domainbarium ferrite published in the literature of H =5900 oe. This lattervalue is based on a coherent rotating model of single domainuninteracting particles when shape and crystal anisotropy are both takeninto account.

Since the ferrite precursor powders produced in accordance with mymethod are typically smaller than the single domain size of mostferrites, it will be appreciated that there is ample opportunity tocontrol the calcining temperature and period to achieve a desiredparticle size. The coreaction of the ferrite precursors and the graingrowth are both time and temperature dependent. The

optimum particle size may vary somewhat with different ferritecompositions but the optimum calcining conditions may be determinedexperimentally as was done in connection with the above example withrespect to barium ferrite.

In general, my method may be employed both to coprecipitate iron withbarium, strontium or lead to form the simple hexagonal ferrites asdescribed and it may be employed to form more complex ferrite-precursormixtures of iron, barium (strontium and lead) and any of a number ofother metals such as manganese, magnesium, zinc, cobalt and the like. Inthese situations the iron and the other metals except for barium,strontium and lead are precipitated as the respective hydroxides, andthe latter elements are precipitated as fatty acid salts. For theprecipitation of barium, strontium and lead long chain saturated orunsaturated fatty acids containing preferably ten to twenty carbon atomsare employed. I consider it desirable to employ a fatty acid of at leastten carbon atoms to assure substantially complete precipitation ofbarium, strontium or lead in the ammonium hydroxide environment. Fattyacids containing more than twenty carbon atoms add little to thecompleteness of the precipitation, and may themselves be difficult todissolve in ammonium hydroxide. Lauric acid and stearic acid areexamples of readily available, substantially pure fatty acids which maybe employed in my process. For best results, it is necessary that onlysufficient fatty acid be used in the coprecipitation process to effectthe precipitation of barium, strontium or lead. All other materialsshould be precipitated as their respective hydroxide salt. I find itnecessary to use the fatty acid salt of barium, cobalt or lead to bringabout a true coprecipitation of these metals with iron hydroxide.However, if all ferrite precursor metals in a given formulation areprecipitated as stearates, which is, of course, usually possible, theprecipitate tends to coalesce and melt during burn out of the fatty acidand the desired ultra fine particle size is lost.

Accordingly, my invention provides a technique of coprecipitating ironand other suitable metals with barium, strontium or lead, The three lastnamed metals are precipitated as long chain fatty acid salts and allother metals are precipitated as the respective hydroxides. Although thepreparation of barium ferrite was described in a specific embodiment,strontium ferrite, lead ferrite and other ferrites may be formed bysubstantially the same process.

Thus, it will be appreciated that other modes of my described methodcould be adapted by one skilled in the art and accordingly the scope ofmy invention should be considered limited only by the following claims.

I claim:

1. A method of forming a ferrite comprising coprecipitating ferrichydroxide and a fatty acid salt of an element selected from the groupconsisting of barium, strontium and lead from an aqueous solution of anammonium hydroxide and a fatty acid to form finely divided particleswherein said iron hydroxide and said fatty acid salt are intimatelymixed, said fatty acid containing at least ten carbon atoms in itsmolecule,

separating the precipitated particles from the aqueous mother liquor,

drying said particles,

burning out the fatty acid organic material from said particles to leavea residue comprising an intimate mixture of ferric oxide and the oxideof said element, and

further heating said particles thereby coreacting said oxides to form aparticulate crystalline ferrite composition comprised of iron, oxygenand an element taken from the group consisting of barium, strontium andlead, said coreacted particles being of substantially the single domainsize of the desired ferrite composition or smaller.

2. A'method of forming a ferrite comprising preparing a first aqueoussolution comprising a suitable salt of iron and of at least one elementselected from the group consisting of barium, strontium and lead,

preparing a second aqueous solution comprising a fatty acid containingfrom ten to twenty carbon atoms in its molecule in aqueous ammoniumhydroxide,

mixing said first and said second aqueous solutions to coprecipitateiron hydroxide with a fatty acid salt of said element to form a finelydivided precipitate wherein said iron hydroxide and said fatty acid saltare intimately mixed,

filtering said precipitate from the mother liquor,

drying the precipitate, burning out the fatty acid organic material fromsaid particles, the residue comprising an intimate mixture of ferricoxide and the oxide of said element, and

calcining said particles to form a crystalline ferrite compositioncomprising iron, oxygen and an element taken from the group consistingof barium, strontium and lead, said particles being of substantially thesingle domain size of the desired ferrite composition or smaller.

3. A method of forming a hexagonal ferrite comprising preparing a firstaqueous solution containing a suitable ferric salt and a salt of atleast one element selected from the group consisting of barium,strontium and lead,

preparing a second solution comprising a fatty acid containing from tento twenty carbon atoms in its molecule in aqueous ammonium hydroxide,the amount of said fatty acid in said second solution beingsubstantially chemically equivalent to the amount of said element insaid first solution, the amount of said ammonium hydroxide being inexcess of the amount which is chemically equivalent to said ferric salt,

mixing said first and said second aqueous solutions to coprecipitateiron hydroxide with the fatty acid salt of said element to form a finelydivided precipitate wherein said iron hydroxide and said fatty acid saltare intimately mixed,

filtering said precipitate from the mother liquor,

drying the precipitate,

burning out the fatty acid organic material from said particles at atemperature of about 200250 C., the residue comprising an intimatemixture of ferric oxide and the oxide of said element, the particle sizeof said particles being 500 angstroms or less, and

- calcining said particles to coreact said ferric oxide and the oxide ofsaid element to form a hexagonal ferrite and to effect the growth ofsaid particles whereby particles of substantially the single domain sizeof the desired hexagonal ferrite composition are obtained, said singledomain size being in the order of one micron.

References Cited UNITED STATES PATENTS 30 ROBERT D. EDMONDS, PrimaryExaminer

